The present invention relates to a method for manufacturing an ethylene-vinyl acetate copolymer (EVA) and an apparatus for the same.
Copolymerization of ethylene with vinyl acetate generates a great deal of heat, which requires efficient removal in the continuous manufacture of EVA. In general, a cooling device such as a jacket and a coil (JandC) is used for absorbing the heat generated. However, in continuous manufacture, polymer scales adhere to a surface of the cooling device, leading to a decrease in its cooling efficiency. Thus, it is necessary to suspend manufacture at regular intervals to remove the scales.
The polymer scales adhere not only to cooling devices that touch the polymerization solution but also to a top of the vessel. The scales may then fall into the solution and cause imperfections such as fish eyes in products, for example films and containers, made from saponified EVA (ethylene vinyl-alcohol copolymer: EVOH).
To reduce these imperfections, JP 11(1999)-116637 A discloses a method for the manufacture of EVA. In this method, a top of the vessel is cooled and the temperature of the top is held lower than that of a polymerization solution (a polymerization temperature). JP 11-116637 A discloses Examples in which the temperature of the top is maintained about 10 degrees lower than the polymerization temperature (65 to 77xc2x0 C. in the Examples). According to this method, the copolymerization is suppressed on the inner surface of the top and this adhesion of polymer scales there is inhibited. However, in this method, the top is cooled only for suppressing the adhesion. Cooling devices are still required to cool the polymerization solution and thus the polymer scales adhere to the surface of such a device.
JP 2(1990)-52922 B discloses another method for the manufacture of EVA. In this method, ethylene discharged from a vessel comes in contact with vinyl acetate (or a vinyl acetate solution) in a heat exchanger and is dissolved into it. The ethylene introduced with the vinyl acetate (solution) into the vessel is evaporated by the heat of polymerization. This vaporization cools the polymerization solution. The vinyl acetate (solution) is used as a solvent for circulating ethylene. According to JP 2-52922 B, the vinyl acetate (solution) should be used, because the condensation of ethylene requires a very low temperature refrigerant. JP 2-52922 B points out that heat absorption by ethylene condensation requires a great deal of ethylene circulated with a booster and that a pressure of ethylene higher than its critical point makes it impossible to condense ethylene.
The method disclosed in JP 2-52922 B employs the heat of condensation of ethylene as well as the heat of dissolution of ethylene to absorb the heat of the reaction and can allow the continuous manufacture of EVA stably over a long period without a jacket or a coil. However, in this method, one of monomers (vinyl acetate) to be supplied into a vessel should be used for the circulation of ethylene. This imposes some restrictions on the method.
First, the method cannot easily be adopted in a batch-type apparatus, because the monomers are not introduced continuously. Such batch-type apparatus is suitable for flexible manufacture of a wide variety of EVA copolymer on a small scale. Second, even in the apparatus for continuous manufacture, the method often causes problems in stability, particularly when operation conditions are forced to change considerably, for example, at the beginning or end of manufacture. In the case of an emergency stop of the operation, the supply of vinyl acetate is suspended, which causes the ethylene to stop circulating as refrigerant.
It was found that a gas existing above the EVA polymerization solution could be condensed at a higher temperature than the condensation point of ethylene. Surprisingly, when the condensate was introduced into the polymerization solution, the solution was cooled more effectively than would be estimated based on the heat of condensation of pure ethylene. The present invention was completed based on this new knowledge.
Viewed from one aspect, the present invention provides a method for manufacturing EVA in a polymerization solution in a vessel. The polymerization solution includes ethylene, vinyl acetate, methanol and a polymerization initiator. This method comprises: cooling a gas including a vapor evaporated from the polymerization solution whereby to produce a condensate of at least a portion of said gas; and introducing said condensate into the polymerization solution whereby to evaporate at least one component (a component or mixture of components) in said condensate. Said vapor comprises said at least one component.
Viewed from another aspect, the present invention provides a method for continuously manufacturing EVA. This method includes the following steps (a) to (d):
(a) continuously introducing ethylene, a vinyl acetate-containing liquid, methanol and a polymerization initiator into a vessel whereby to form a polymerization solution in the vessel;
(b) continuously cooling a gas that includes a vapor evaporated from the polymerization solution whereby to produce a condensate of at least a portion of said gas;
(c) continuously introducing the condensate into the polymerization solution whereby to evaporate at least one component in said condensate, wherein said at least one component (a component or mixture of components) is included in said vapor in step (b); and
(d) continuously discharging a portion of the polymerization solution from the vessel.
As the vinyl acetate-containing liquid, vinyl acetate or a vinyl acetate solution can be used. The vinyl acetate solution is preferably a vinyl acetate solution in alcohol, more preferably a vinyl acetate solution in methanol.
The present invention can allow the stable manufacture of EVA over a long period even if the supply of monomers is stopped or suspended. This method can be applied to batch-type apparatus as well as to continuous operation-type apparatus. The condensate of the gas provides sufficiently large cooling effect to eliminate the need for direct cooling of the polymerization solution. Thus, the method is advantageous in suppressing the formation of polymer scales. As is evident from a large cooling effect observed and from the high condensation temperature of the gas, it is believed that the condensate is not produced from pure ethylene. Probably, cooling the gas causes some of its components other than ethylene, for example vinyl acetate and/or methanol, to condense before the ethylene condenses. The ethylene in the gas would then be dissolved into the liquefied ingredients, forming a condensate of the gas.
Viewed from still another aspect, the present invention provides an apparatus for manufacturing EVA. This apparatus includes a vessel for containing a polymerization solution comprising ethylene, vinyl acetate, methanol and a polymerization initiator, and a cooling device. In this apparatus, the cooling device is placed in a position such that the cooling device contacts a vapor evaporated from the polymerization solution but does not contact the polymerization solution.
The cooling device may be placed inside or outside the vessel. When the cooling device is positioned inside the vessel, the xe2x80x9cposition such that the cooling device does not contact the polymerization solutionxe2x80x9d will depend on the design of the vessel, but in general the position is preferably 50% or higher, more preferably 60% or higher, of the inner height of the vessel. Thus, the cooling device can be placed in the vessel at 50% or higher of the inner height of the vessel, or outside the vessel so as not to contact the polymerization solution.
The cooling device is placed so as to condense the gas including a vapor evaporated from the polymerization solution. When a cooling device is in contact with the polymerization solution, almost all of the cooling force of the device is expended on the solution as that has a higher heat transfer coefficient. The heat exchanger disclosed in JP 2-52922 B directly cools vinyl acetate that flows along the walls of the tubes in the heat exchanger, rather than directly cooling the ethylene. This device is suitable for dissolving the gas into a solvent, but is not suitable for producing a xe2x80x9ccondensatexe2x80x9d of the gas. JP 11-116637 A discloses a cooling device that contacts a top of the vessel rather than the vapor evaporated from the polymerization solution. To condense the gas on the wide top of the vessel would require a cooling device with an excessively high cooling ability. As is evident in JP 11-116637 A, the top of the vessel, which has a large heat capacity, can be cooled such that the adhesion of polymer scales is suppressed, but the temperature of the top does not fall low enough to condense.